The present invention relates to the plating of copper onto zinc substrates or the like. More particularly, the present invention relates to a method for plating of Cu.sup.+1 (copper I or cuprous) ions out of a Cu.sup.+2 (copper II or cupric) bath from the cathode diffusion layer.
Zinc die castings are common in consumer and automotive applications. Many times it is desirable to plate chromium plate over zinc die castings or the like. Particularly in such situations, it is necessary to provide a strike of copper and/or nickel as an undercoat to a chromium plating operation. The quality of the resulting chromium plate is directly proportional to the adherency of the underlying copper strike.
It has long been known that use of copper cyanide electrolytes provides good plating results with adherent copper deposits on a zinc substrate. Because of the existence of cyanide in these solutions, several environmental disadvantages have been found. For instance, if the pH becomes too low in the bath, hydrogen cyanide gas may be produced, causing a hazard to workers or the like. Additionally, the by-products of such baths are highly toxic, creating hazardous waste disposal problems. Thus, there has been a long sought need in the art to replace cyanide copper plating with cyanide-free processes while maintaining the desirable characteristics of copper plate on zinc substrates.
Many cupric based processes which attempt to remove the cyanide electrolyte and replace it with less toxic copper strikes have been proposed. Typically, these are organophosphonate-type baths and provide limited results, provided the chemistry is watched very closely. Examples of these include: U.S. Pat. No. 4,521,282, entitled "Cyanide-Free Copper Electrolyte and Process"; U.S. Pat. No. 4,469,569, entitled "Cyanide-Free Copper Plating Process"; and U.S. Pat. No. 4,462,874, entitled "Cyanide-Free Copper Plating Process", all of which are commonly assigned to the present assignee. While these processes have proven commercially effective, there still remains a need to provide more adherent copper strikes on zinc substrates in a cyanide-free process. One process which has been proposed to duplicate the Cu.sup.+1 plating of cyanide solutions is that set forth in U.S. Pat. No. 5,750,018, entitled "Cyanide-Free Monovalent Copper Electroplating Solutions". This patent discloses a cuprous copper bath by utilizing strong reducing agents and low pH phosphate or pyrophosphate based copper electrolytes. By utilizing these strong reducing agents, a Cu.sup.+1 ion is plated onto the zinc, creating a strong copper strike. While such baths may prove useful in creating a copper plate having cyanide bath-type properties, such baths are inherently unstable due to the nature of the copper ion.
Copper ions have an affinity to a Cu.sup.+2 valence. Thus, in order to maintain the Cu.sup.+1 valence found in the '018 patent, the bath chemistry must be carefully monitored and adjusted to maintain these parameters. Additives quickly break down in such low pH conditions, further complicating maintenance of the baths. Therefore, from a commercial plating viewpoint, this process is undesirable because it requires very high maintenance and close monitoring to maintain a relatively unstable Cu.sup.+1 bath.
Therefore, there remains a need in the art to provide a suitable cyanide-free copper process for plating over zinc die cast. Preferably, the copper strike plated from such a process has the physical adherency properties which are close to or equivalent to cyanide copper-type strikes.